When a change is made to a page, the MediaWiki software saves the change to the main database. The next time a non-logged in user views the page, a copy of that page is made to a "server cache" on a set of secondary servers. Until the page is changed again, all non-logged in users who try to view that page will be shown the same copy from the server cache. That way, users who are viewing but not editing do not put a drain on the main database server, and still see the most recent version of the page.

Normally, when a change is made, the page is marked as "touched" and the software will create a new copy of the page for the server cache. However, when a change is made to a subpage or template, but not to the article page which transcludes it, the article page is not marked as "touched," and a new copy is not generated for the cache. Therefore anyone viewing the article page may see a cached page which includes old versions of the subpage or template.

The purge command forces the server cache to purge the old version of the page. The next time the page is viewed, the page will be rebuilt from scratch, including fresh versions of all subpages and templates, and a new copy will be made for the server cache. Note that this may take a few minutes in extreme cases.

Purging an image's description page can be used to force the software to regenerate all the thumbnails for that particular image. If an image thumbnail will not regenerate successfully after the original image has been purged, it is probably because the server has cached an error page with the URL of a thumbnail, but the page is not purged because the actual thumbnail does not exist. The solution is to request the thumbnail with an unusual URL, say by appending ?1 to the end. The thumbnail should then be successfully generated. Finally, do action=purge to delete the cached error. You should only do this if you have tried an ordinary action=purge and it hasn't worked.